Leetcode: Binary Tree Postorder Traversal

思路:

1. 堆栈模拟

总结:

1. 使用堆栈的话, 后续遍历和前序遍历并不是对称的, 和递归函数的写法大不相同

代码:

自己最初写的代码, 遍历一遍后, 树已经被破坏掉了 

class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> res;
		if(root == NULL)
			return res;

		stack<TreeNode*> stc;
		stc.push(root);
		while(!stc.empty()) {
			TreeNode* node = stc.top();
			if(node->left == NULL && node->right == NULL) {
				res.push_back(node->val);
				stc.pop();
			}
			if(node->right != NULL) {
				stc.push(node->right);
				node->right = NULL;
			}
			if(node->left != NULL) {
				stc.push(node->left);
				node->left = NULL;
			}
		}
		return res;
    }
};

　　

参考别人的代码, 做的改进

class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> res;
		if(root == NULL)
			return res;
		vector<TreeNode*> stc;
		stc.push_back(root);
		while(!stc.empty())	{
			TreeNode* node = stc.back();
			res.push_back(node->val);
			stc.pop_back();
			if(node->left != NULL) {
				stc.push_back(node->left);
			}
			if(node->right != NULL) {
				stc.push_back(node->right);
			}
		}
		reverse(res.begin(), res.end());
		return res;
    }
};